1. Field of Invention
The present invention relates to a load driving apparatus for driving the load of the electrical equipment of a vehicle: brake, heater or the like.
2. Description of Related Art
The load driving apparatus disclosed in Japanese Patent Application No. H5-168164 comprises a high-voltage battery connected in the form of loop, to a headlight, an FET, a variable duty ratio PWM control circuit, a voltage detecting circuit and a switch. The duty ratio of the variable duty ratio PWM control circuit is set so that the work load of a headlight rated 12[V] when a high-voltage power supply of 24[V] is applied becomes equal to that of a normal 12[V] power supply. Based on the duty ratio set, the FET switching is controlled, thus controlling the work load. Any change in voltage of the high-voltage power supply is detected by the voltage detecting circuit, and the duty ratio is corrected based on the detected voltage change.
In the above load driving apparatus, however, a low-voltage rated load is used without alteration in a high-voltage circuit. Therefore, a problem is encountered in the increased fluctuation amplitude of the current supplied to the load rated low-voltage and the generated noise.
Accordingly, it is a primary object of the present invention to provide a load driving apparatus capable of reducing the noise of the current supplied to the load where a low-voltage rated conventional load is driven by a high-voltage power supply.
To achieve the above object, the load driving apparatus of the present invention comprises a power supply, a load circuit, a first switch, a limiting resistor, a second switch and controller for controlling the wave form of the power supply voltage.
The power supply voltage is applied to the load circuit by a power supply, and the load is driven. The first switch is connected to the load circuit and the power supply in series, and starts or stops the application of the power supply voltage to the load circuit. The limiting resistor is connected to the load circuit in series, dividing the power supply voltage between itself and the load circuit, and thereby limiting the current supplied to the load circuit. The second switch is connected to the limiting resistor in series and connected to the first switch in parallel, and starts or stops the application of the power supply voltage to the load circuit and the limiting resistor. The controller opens or closes the first switch and the second switch, thereby applying power supply voltage of a plurality of values to the load circuit, and controlling the wave form of the power supply voltage.
According to the above structure, when the first switch is opened, the power supply voltage from the power supply is applied to the load circuit without alteration. If the first switch is closed and the second switch is opened, the divided power supply voltage is supplied to the load circuit. The state in which the power supply voltage is applied without alteration (former state) and the state in which the divided power supply voltage is applied (latter state) are repeated, thus controlling the wave form of the power supply voltage.
Controlling means shortening the period during which the second switch is opened and the first switch is closed, applying the power supply voltage divided by the limiting resistor to the load circuit, as compared with a period during which the first switch is opened, applying the power supply voltage of the power supply to the load circuit.